Plasma generator



v Marchll, 1958 J. s. FOSTER, JR

PLASMA GENERATOR Filed June 2, 1955 zOEbmm INVENTOR. I JOHN S. FOSTER,JR.

BY fiMd 0 AZ'TORNEY.

. ciency is low.

United States atentO PLASMA GENERATOR John- S. Foster, Jr., Livermore,caurl, assignor to-the' United States of America as represented by theUnited StatesAtomic Energy Commission Application June 2, 1955, SerialNo; 512,904 8 Claims. (c1. 3.13:7

Th-is .Sinvention relates to apparatus fonprodiicing ioniz'ed gas. and,more specifically, to. a generator for prowise.

'In conventional; practice the-plasmais usually. accomlisionprocessesofthe. neutral. gas particles and other attendant phenomenonseriously alter the behavior of lthe plasma and may, rendersuchawplasmaunfit for varioususes. Power requirements irnconventionalmethods-of plasma production are high andplasma production efli- Inconventionall-practice a plasma-may be generatedby electrombombardmentof. gas molecules as in an arc discharge. However, ,an electricallyneutral ionized gasdischarge has been very difiicult to. obtain.

The device of the present invention providesanuncontamitiatedpalsmanotassociated Wuhan electron beam and accomplishnes thisresult-byinducting ionization. of gas particles in a restricted regionand: selectively extracting'the plasma particles through anorificeoflimited size. Since the plasmais selectively extracted,contaminationby, neutral particles is largely eliminated and there is'no association with an electron beam. The selective extraction iseffected'by utilizing an axially symmetric magnetiofield acting, incooperation with a pressure. differential established through .saidorifice whereby the probability of a plasma particle escapingthrough.orificef is h'igh'compared tothat .of .a neutral particle. High"ionization efii'cie'ncy, and maximum power transfer is" ohtainedlbyutilizing very high radio frequency generatin'gi'means'efilcientlycoupled toa plasma. generation chamber by establishing and'maintainingcertain critical relationships between the electron plasma frequency,cyclotron frequency of the electrons in the generating chamber, and theoperating frequency of the radio fre quency energizing source.

It is therefore an object of the present inventionato provide methodsand apparatus for generating a plasma.

Another object of the invention is. to: provide a-generator capable ofproducing a plasma'uncontaminatedby an electron beam or neutralparticles. I

An additional objectlof the-inventionlis toprovid -a plasma generatorwherein radio frequencyjenergy: is :employed in the production ofions ofa plasma.

Atfurther object of:the:invention. is 'to..provide a generatorconsuming'minimum-powerin the productionof aplasmaa.

A. stillfurther object .of-the. invention is;.to provide-aplasmagenerator utilizing: an axially symmetric mag- .uetic fieldactingin cooperation with a pressure difieren-= panie'd'by, antenergeticelectron..beam,.and the plasma is highlyflcontaminatedwith neutral gasparticles; Colr 2,826,708 Patented Mar. 11, 1958 lie tial'to selectivelyextr'act'and focus a plasma through an orifice.

The invention, both as to its organization and method of' operation,together with further objects and advantages thereof,lwill best beunderstood by reference to the followingn'specification taken inconjunction with the accompanyingdrawing, of which:

, Figure 1 is a longitudinalcross sectional view of a preferredembodiment. of'th'e invention; and

Figure 21is'a cross section taken along plane 2-201? Figure-.1.

The plasma generator of the invention is considered suitable. forproviding a plasma to be' utilized in a variety of devicescharacteristically similar in that aplasma, ii e.,-ionized gas,.isemployed" therein for sundry purposes. Electrically neutral'plasmasareof interest in the thermonuclear. fields and for experimentalpurposes;

'pla'sm'a' generator ofthe invention will include an evacuated systeminto which the plasma is discharged and utilized for diverseexperimental purposes.

In'brief, therefore," the plasma generator of the invention' includesmeans providing anevacuated system in which thereis contained a region;wherein the plasma is to be'utilizedand also a plasma generation chamberhaving an orificecommunicating with the evacuated sys tem: In thischamberthere is produced animpure plasma contaminated by neutralmolecules; For producing said impure plasma there is provided meansfor:introducing a suitable gas to the plasma generation'chamberand therebyestablishing a gas pressure difierentialacross said orifice, means forestablishing an axially symmetric magnetic fieldnormal-to said orifice,and very high radio frequency generating means-adapted to energize'the'plasma generationchamber-causingionizationof the gasithereinz The alonga a path most" favorable for extraction through the orifice;Theextractionof an electrically neutral'uncontaminatedplasma'isaccomplished by the focusing action Of-"th e magnetic field cooperatingwith the'pressure differential establishedacrossthe orifice, as will bemore fully disclosed hereinafter: I

More particularly, referring to the accompanying drawing,- the meansproviding an evacuated system may comprisea'n elongated evacuatedcylindricalhousing 11 coupled as by a' conduit tube'12' to'aconventional vacuum pump (not shown); Conveniently, such a housing 11is-constructed of a 'tubular-section 13 flanged at the extremitiesforthe' attachment of end cover plates 14 and-16=a's:-bymeans'of flangebolts'17 and 18. Annular vacuum sealing elements '19 and 21-are disposedbetween the end plates and the flangedextremities of the housing 11 mprovide. a vacuum sealth'erebetween. The housing'll'should beconstructed of material-perviousto'a magnetic field as will. be apparentfrom the context'of the specification.

lt will' be'noted here, in the event the plasma generator is used in anapparatus such as anaccelerator or the like, thatthehousing'll may beeliminated and an evacuated regionof" such apparatus may provide thehercinbeforementioned means providing an evacuated system. Itwill'also'b'e appreciated that the plate 16 located'attheiorwardjextremity'of'the housing 1Imay be eliminated to enable'thehousing to be secured directly to such apparatus when" the plasmagenerator is used therewith. For particular applications of theinvention 3 a other means providing an evacuated system will be obvious.

To provide a most convenient and simplified constructionathe plasmageneration chamber-may be constructed as an integral extension portionof a tubular wave guide 22 whereby the wave guide is made to serve themultiple purposes of providing structural portions of the said chamberand means for coupling the ionization energy source thereto. Morespecifically, the wave guide 22 is provided with a terminal flange 23for the attachment of a cup-shaped terminal chamber member 24 having atubular wall portion 26 of size and shape similar to said wave guide, anend face 27, and provided after. A D. C. power supply (not shown) isprovided with a flange 28 attachable as by means of bolts 29. to

the terminal flange 23 of the wave. guide "22. The periphery of a gastight partition 31 formed of electrically non-conductor'material isdisposed between said flanges 23 and 28 so as to separate the plasmageneration chamber proper 24 from the remainder of the wave guide 22,and a sealing element 32 may be disposed outwardly therefrombetween theflanges to provide a more positive vacuum seal.

The end face 27 of the chamber member 24 is centrally apertured toreceive an exit tube 33 provdiing an orifice 34 axially aligned withsaid wave guide 22. Several structural arrangementse may be employedwhereby said orifice is made to communicate with the interior of saidhousing 11. .For structural convenience it is preferred that the waveguide be disposed so as .to pierce or be supported in vacuum tightfashion by one of said cover plates, e. g., plate 14, whereby the waveguide 22 and orifice 34 are in approximate axial alignment with thehousing 11.

within an aperture 36 provided in said plate 14 and extending within thehousing 11; however, if desired the plasma chamber 24 may be terminatedat the plate 14 and with only the tube 33 piercing said plate.

For the purposes of the invention, the wave guide 22 is constructed andproportioned in accordance with conventional practice with regards totubular transmission lines. While a rectangular guide is shown othertubular guidesmay also be employed. However, for reasons apparenthereinafter the material employed while being necessarily an electricalconductor must also be pervious to a magnetic field, i. e., anon-magnetic electrical conductor is employed in constructing the waveguide and plasma chamber. Also, to minimize heating of the partition -31the partition is disposed at a minimumcurrent node in the wave guide'22, i. e., the longitudinal length of the plasma generation chamber 24is preferably made a half-wave length or multiple thereof.

The hereinbefore mentioned means for introducing gas tothe plasmageneration chamber may comprise a conduit=37 communicating with anexternal gas supply 38 andterminating in the plasma generation chamber24 whereby a pressure differential is established across the orifice 34by introducing gas therethrough. Gaseous material such as hydrogen,hydrogen isotopes or any other desired gaseous material may be employed,as required for the desired purpose. The conduit 37 as shown in Figure 1transpierces the plate 14 in gas tight fashion ical structure.

The means for establishing an axially symmetric magnetic field normal tosaid orifice may comprise a solenoid 43 disposed coaxially about thehousing 11 and extending along the longitudinal axis thereof a distancesuflicient to provide an axially symmetric magnetic field in both theplasma generation chamber 24 and tthe evacuated region of the housing11. Specific boundaries of the magnetic field will become more apparentherein- For the most compact arrangement the wave guide 22 is disposedin vacuum tight relation to energize the solenoid 43.

Very high radio frequency energy required to energize said plasmageneration chamber can be supplied by various conventional means and maycomprise a continuous wave magnetron oscillator 44 coupled to the waveguide 22 as through a matching wave guide section 46. It will beappreciated that any appropriate electrical oscillatory apparatusoperating in the microwave region may also be employed such as klystronsand the like. The electromagnetic waves propogating within the waveguide 22 energize the free electrons in the plasma generation chamber23. Stray electrons always exist in such a system and it is well knownin the art that an energetic electron moving in a magnetic field willfollow a helical path, the axis of which lies perpendicular to thedirection of rotation. Thus the energized electronswill describe aroughly tight helical path and produce ionizati-onas they collide withthe gas molecules and a plasma will be thereby formed in the plasmageneration chamber 24. V Moreover, the ionized gas molecules beingpositively ionized will describe generally similar paths, with oppositedirection of rotation to the electrons and will form with the electronsan electrical neutral plasma in the chamber. A v f To begin operation ofthe plasma generator of the invention, the housing 11 is continuouslyevacuated by said vacuum pump whereby continuous evacuation of theplasmageneration chamber also is efiected. Gas such as deuterium,tritium, and the like from which the plasma is to be generated is thencontinuously introduced into the generation chamber 24 from the gassupply 33 and at a rate correlated with the ratio of evacuation toestablish the desired operating pressure in said chamber and toestablish and maintain a gas pressure differential across the orifice34. f Actuation of the radio frequency energizing means 44 then suppliesradio frequency energy to the wave guide 22 wherein the electromagneticwaves propagate to the plasma generation chamber 24. Electrons presenttherein are caused to accelerate in the aforesaid helical paths by theinteraction of electric fields produced by said radio frequency energyand the constraint of said axially magnetic fields producing copiousionization of the gas molecules thereby forming an impure plasma, i. e.,neutral atom contaminated electrically neutral gas in the chamber. Also,both the electrons and positive ions are constrained by said axiallysymmetric magnetic field t-o helical paths I of which the axes areparallel to the axis of the chamber .11 and said orifice 34.

Several parameters are mutually interrelated and tend to beinterdependent in the operation of the plasma generation chamber. Formaximum efi'iciency the magnetic fieldthat passes through the plasmageneration chamber 24 is adjusted so that the cyclotron frequency forelectrons (Fc) in the chamber is equal to that of the operatingfrequency (Fm) of the'magnetron as given by the following relation:

where:

Under-these conditions, electrons in the plasma gen- -eration chamber24receive the major portion of the energy from the electrical field, anda plasma'can'be produced with the least energy. Withappropriately adtheion" density (n) will increase until the electron plasma -justedt powerinput and gas pressure, in, the plasma,

"frequency (Fp) approaches the :mag-netron frequency {(Fm) as indicatedby the followingr'ela'tion:

the focusing effect of the axially symmetric magnetic field acting incooperation with the flow of gas through the orifice caused by the lowerpressure (below approximately mm.) in the evacuated region contained bythe housing 11. The gas flow is essential to the production of theelectrical neutral plasma since the electrical extraction methodsemployed conventionally cause charge separation and the production of apositive ion beam. It will be appreciated that the short section oftubing 33 is provided to reduce the rate of gas flow from the plasmageneration chamber 24 establishing the indicated pressure difierentialthereacross. Due to the constraint afforded by the magnetic field theorifice structure will have little effect on the plasma.

It will be noted that the aforementioned extraction method effectivelyscreens out most of the neutral particles as the plasma is selectivelyfocused and flows through the orifice 34. The probability that a neutralparticle following its usual random path has of escaping through such anorifice is small compared to that of a plasma particle focused asdescribed by an axially symmetric magnetic field. The few gas moleculesthat escape through the orifice will diffuse randomly into the evacuatedregion as they are not effected by the magnetic field therein, and thusthe extracted focused plasma is further freed from contamination byneutral particles as it emerges from the orifice into the housing. Inaddition, the electron beam usually associated with the production of aplasma is eliminated in the present invention by the extraction methodemployed; nevertheless, the electrons required to provide electricalneutrality are entrained and carried along by the positive ion contentof the plasma. Hence, in the low pressure region of the housing 11 thereis provided a low temperature uncontaminated plasma, focused along theaxis of the magnetic field, without the presence of an electron beam orneutral particles and which can be utilized for various purposestherein.

While the invention has been disclosed with respect to a singlepreferred embodiment, it will be apparent to those skilled in the artthat numerous variations and modifications may be made within the spiritand scope of the invention and thus it is not intended to limit theinvention except as defined in the following claims.

What is claimed is:

l. A plasma generator comprising in combination a plasma generationchamber having an exit orifice, a source for introducing gas to saidchamber, means for ionizing gas in said chamber wherein a plasma isformed, magnetic field means for focusing said plasma, and means forestablishing a pressure differential across said orifice whereby saidplasma is focused and extracted through said orifice by said magneticfield in cooperation with said pressure differential.

2. A plasma generator comprising in combination an evacuated housing, aplasma generation chamber having an exit orifice communicating with saidevacuated housing, a source for introducing gas to said chamber, thereby:festablishing :a pressure :idifierential racross sai'd orifice,

means establishing :an axially asymmetric :magnetic F. field :ncrrnaltorsaid orifice,aandameanslfor:establishingthigh s'frequency electricalifields 2 in said rchamber :for-i-onizing sgas in said chamber ito :form:;a plasma that :is :focused 13.1111 :extractdtthroughnsaid -;orificeby' zsaidfmagnetict: field :actingain cooperation withusaidrpressuredifferential.

13. 1A ,plasma .1 generator comprising in 1 combination tan evacuated.housing, ;a plasma -,generation chamber having an :exit :orificeicommunicat-ing with .said evacuated l1ousing, a variable :pressuresource1for introducing :gas -to .said :,chamber zthereby establishing;:a' pressure differ- -.enti'al across :said .x'mrifice, magneticJzfield .means :estabdishing an =ax-ially asymmetric magnetic fieldnormal 1 to said orifice of controlled intensity, arid-.meanstproducing::and coupling thigh zfrequency 1 electromagnetic ,rradiration. into 1said chambjer=with theafrequency thereof :substantially equal to thecyclotron frequency of electrons in said chamber and to the electronplasma frequency for maximized ionization efficiency, whereby a plasmais formed in said chamber and is magnetically focused and pressurepropelled through said orifice.

4. A plasma generator comprising in combination an evacuated housing, aplasma generation chamber having an exit orifice communicating with saidevacuated housing, a source for introducing gas to said chamber therebyestablishing a pressure differential across said orifice, a solenoidelement adapted for energization to establish a magnetic field directedaxially through said orifice, and wave guide means terminating in saidchamber whereby very high radio frequency excitation applied to saidwave guide excites and ionizes gas molecules in said chamber producing aplasma which thenceforth is focused and extracted through said orificeby said magnetic field acting in cooperation with said pressure differential.

5. A plasma generator comprising in combination an evacuated housing, awave guide element having an exit orifice communicating with saidevacuated housing, a gas tight partition pervious to radio frequencyradiation disposed within said wave guide forming a plasma generationchamber proximal to said exit orifice, a source for introducing gas tosaid chamber thereby establishing a pressure differential across saidorifice, means for establishing an axially symmetric magnetic fieldnormal to said orifice, and high frequency generating means energizingsaid wave guide to cause ionization of gas in said plasma generationchamber wherein a plasma may be formed and thenceforth focused andextracted through said orifice by said magnetic field acting incooperation with said pressure differential.

6. A plasma generator comprising in combination an evacuated housing, awave guide transpiercing one wall of saidevacuated housing, said waveguide having an exit orifice opening into said evacuated housing, a gastight partition pervious to radio frequency disposed within the endregion of said wave guide extending into said evacuated housing forminga plasma generation chamber proximal to said exit orifice, a source forintroducing gas to said chamber thereby establishing a pressuredifferential across said orifice, magnetic field means for establishingan axially symmetric magnetic field normal to said orifice, and radiofrequency generating means coupled to said wave guide for ionizing gasin said chamber wherein a plasma may be formed and thenceforth befocused and extracted through said orifice by said magnetic field actingin cooperation with said pressure differential.

7. A plasma generator comprising in combination an .wave guide proximaltossaid exit orifice thereby forming I a plasma generation chambercommunicatingiwithsaid vessel, a source for introducing gas to saidchamber thereby establishing a'pressure difierential across saidorifice, a solenoid disposed coaxially with said vessel to provide anaxially symmetric magnetic field in said vessel and said chamber normalto said orifice, generating means for producing high frequencyelectromagnetic radiation coupled to said wave guide through a matchingsection whereby said wave guide is energized causing ionization of thegas molecules in said plasma generation chamber to establish a plasmatherein, which plasma is focused and extracted through said orifice bysaid magnetic field acting in cooperation with said pressuredifferential.

8. In a method for providing an uncontaminated plasma, the stepscomprising producing a contaminated plas- 10 plasma beam.

ma in a defined region having an exit orifice, and extracting saidplasma by focusing said plasma along the axis of said orifice with. anaxially symmetric magnetic field and simultaneously providing adecreasing gas pressure gradient across said orifice whereby said plasmais selectively extracted from said'defined region through said orificeas a Well defined plasma beam and any neutral gas particles emergingfrom said orifice diffuse in a randomly indiscrete manner so as toseparate from said References Cited in the file of this patent UNITEDSTATES PATENTS 1 2,400,557 Lawlor May 21, 1946 2,545,595 Alvarez, Mar.20, 1951 2,582,216 -Koppius Jan. 15, 1952

